Conventionally, Device-to-device (D2D) communication has been a technique for improving spectral utilization, satisfying increasing demand for local traffic load and providing better user experiences. D2D communication could be defined as two peer devices in direct communication with each other without relying on a cellular network to relay user data in between. D2D communication may operate on a stand-alone spectrum which could be categorized as outband D2D. D2D communication may also coexist with cellular networks within a cellular spectrum which is categorized as inband D2D.
A motivation for choosing inband D2D is better control of the radio resources of a licensed cellular spectrum. Inband D2D may further be categorized into underlay and overlay operations. In the underlay scenario, D2D links share the same radio resources with the cellular links. In the overlay scenario, dedicated radio resources not occupied by the cellular links are allocated to the D2D links. An advantage of the inband overlay D2D operation is that there would normally be no interferences between D2D and cellular links. However, the overlay D2D operation would rely on centralized allocation, such as allocations by a base station, of a radio resource block (RB) to either D2D or cellular users. Challenges related to D2D communications in cellular networks as well as cellular network assisted D2D communications were abound and were discussed.
Overlay D2D communications would allow D2D and cellular users to use separate RBs and thus is not as efficient as the underlay D2D communications in terms of spectrum utilization. For better spectrum utilization, underlay D2D communications would usually be preferred, and thus the potential of underlay D2D communications for improving spectral efficiencies of cellular networks by offloading traffic from the cellular network to the D2D links has been investigated. However, underlay D2D communications may cause interferences to the cellular network which could severely degrade the performance of the cellular users. Thus, interference management and efficient resource reuse schemes could be critical issues. Subsequently, a practical and efficient scheme has been proposed for generating local awareness of interferences between the cellular and D2D terminals at a base station (BS), which then exploits the multiuser diversity inherent in the cellular network to minimize the interferences. Also, throughput could be maximized for a network with a single D2D pair and a single cellular user while the quality-of-service (QoS) of the cellular user was considered.
For effective interference management, the central unit such as a BS that allocates the radio resources would usually need to know the locations of users as well as the channels between D2D and cellular users. Information related to channels between D2D users in general could be quite difficult to obtain. In addition, optimizations of the network in the presence of mutual interferences would typically be computationally intensive. Thus, in contrast to overlay D2D, effective operations of underlay D2D communications would incur more system overhead and would be more difficult. Operations involving overlay D2D in which D2D and cellular users use orthogonal RBs and underlay D2D in which D2D and cellular users use non-orthogonal RBs were considered.
To facilitate inband D2D in cellular networks with less interference between D2D and cellular users, many existing works assumed that D2D communication operates in the cellular uplink (UL) band. This has also been adopted as the basic assumption in the 3GPP D2D design.